In the case of sk->dccps_qpolicy == DCCPQ_POLICY_PRIO, dccp_qpolicy_full
will drop a skb when qpolicy is full. And the lock in dccp_sendmsg is
released before sock_alloc_send_skb and then relocked after
sock_alloc_send_skb. The following conditions may lead dccp_qpolicy_push
to add skb to an already full sk_write_queue:

thread1--->lock
thread1--->dccp_qpolicy_full: queue is full. drop a skb
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_full: queue is not full. no need to drop.
thread2--->unlock
thread1--->lock
thread1--->dccp_qpolicy_push: add a skb. queue is full.
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_push: add a skb!
thread2--->unlock

Fix this by moving dccp_qpolicy_full.

Fixes: b1308dc015eb ("[DCCP]: Set TX Queue Length Bounds via Sysctl")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
---

v2: 
1. call dccp_qpolicy_full first after relocking.
2. change "Fixes:" tag.

 net/dccp/proto.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index eb8e128e43e8..e13641c65f88 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -736,11 +736,6 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	lock_sock(sk);
 
-	if (dccp_qpolicy_full(sk)) {
-		rc = -EAGAIN;
-		goto out_release;
-	}
-
 	timeo = sock_sndtimeo(sk, noblock);
 
 	/*
@@ -759,6 +754,11 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (skb == NULL)
 		goto out_release;
 
+	if (dccp_qpolicy_full(sk)) {
+		rc = -EAGAIN;
+		goto out_discard;
+	}
+
 	if (sk->sk_state == DCCP_CLOSED) {
 		rc = -ENOTCONN;
 		goto out_discard;
-- 
2.25.1

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Fri, 29 Jul 2022 19:00:27 +0800 you wrote:
> In the case of sk->dccps_qpolicy == DCCPQ_POLICY_PRIO, dccp_qpolicy_full
> will drop a skb when qpolicy is full. And the lock in dccp_sendmsg is
> released before sock_alloc_send_skb and then relocked after
> sock_alloc_send_skb. The following conditions may lead dccp_qpolicy_push
> to add skb to an already full sk_write_queue:
> 
> thread1--->lock
> thread1--->dccp_qpolicy_full: queue is full. drop a skb
> thread1--->unlock
> thread2--->lock
> thread2--->dccp_qpolicy_full: queue is not full. no need to drop.
> thread2--->unlock
> thread1--->lock
> thread1--->dccp_qpolicy_push: add a skb. queue is full.
> thread1--->unlock
> thread2--->lock
> thread2--->dccp_qpolicy_push: add a skb!
> thread2--->unlock
> 
> [...]

Here is the summary with links:
  - [v2] dccp: put dccp_qpolicy_full() and dccp_qpolicy_push() in the same lock
    https://git.kernel.org/netdev/net/c/a41b17ff9dac

You are awesome, thank you!
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